Optical harmonic generation using nonlinear medium such as a crystal, provides a method for doubling, tripling or more generally producing the Nth harmonic of the frequency of electromagnetic radiation emitted by a laser or other high intensity source. The 3rd harmonic is produced by sum frequency generation of the laser fundamental and its 2nd harmonic beams through 2nd order nonlinearities of the nonlinear medium (P( )=.sub.0 (3)*P.sub.0 ( )*P.sub.0 (2)). The common 3rd harmonic generation method, which is known in the art, combines the fundamental and the 2nd harmonic beam in a nonlinear crystal to generate its 3rd harmonic in the external laser cavity. For a high power laser, typical efficiency is limited to less than 25% of overall conversion efficiency, since unconverted fundamental beam and 2nd harmonic beams are wasted in that configuration. The overall conversion efficiency=P(3)/P.sub.0 ( )=.sub.0 (3)*.sub.1 (2)P.sub.0.sup.3 ( )) is less than 25% for even very high power pulsed Nd:YAG laser (see Continuum Product brochures).
Recent intracavity 3rd harmonic has improved conversion efficiency substantially. See for example U.S. Pat. No. 5,898,717. But unconverted 2nd harmonic is still wasted in those configurations.
The theory of intracavity 2nd harmonic generation (SHG) has been discussed by R. G. Smith, in the I.E.E.E. Journal of Quantrum Electr. QE-6, 215-223 (1970). Harmonic generations have been demonstrated in crystals such as LiNbo.sub.3, BaNa (NbO.sub.3); Lio.sub.3, KDP, KTiOP.sub.4, BBO, LiB.sub.3 O.sub.5, and CsLiB.sub.6 O.sub.10. LiB.sub.3 O.sub.5 is also referred to as LBO crystal and CsLiB.sub.6 O.sub.10 is referred to as CLBO.
4th harmonic generation is known in the art. See U.S. Pat. No. 5,206,868. Resonance cavity is built to produce harmonics for single longitudinal mode has been demonstrated. However, the cavity is rather complicated and sometime the device is not practically useful. Multi-longitudinal mode lasers are more popular and easier to maintain. 5th harmonic generation is also known see: U.S. Pat. No. 5,742,626. Improved efficiency and more practical harmonic generations are sought for harmonic generation lasers.